Fluke 189 Help Needed

[aueloxdad]

I have a gently used Fluke 189 that I just recently wanted to test a 24VAC relay and was surprised by the lack of voltage reading.  Fuses are good, MOV's are open PTC @1.1kohm's.  It will read DC voltage, resistance and will read Hz.  Any thoughts?  Thanks in advance.

[bdunham7]

Can you measure some other AC source, perhaps mains voltage?  I take it the issue is that it reads near zero volts on the AC range?

[aueloxdad]

I have checked on a 120 volt circuit as well.  Just the "open leads" indication.

[bdunham7]

I have checked on a 120 volt circuit as well.  Just the "open leads" indication.

Whoa, wait a minute--what does that mean?  Does it give you a value near zero with a hundred or so counts of noise in the last few digits--or does it say "LEAdS" ?

[BeBuLamar]

Open Lead you meant OL? In that case try manual ranging by pressing the range button a few time. If it works then somehow the autoranging doesn't work.

[aueloxdad]

"OL" is correct.  Changing from auto range to manual has no effect.

[bdunham7]

OK, so if you put it in the VAC mode--the first detent after "OFF", with the leads disconnected (or not installed) you get "OL" (which means overload, not open leads) on the display?  How about if you short the inputs with a banana cable or by shorting the two test leads together--what does it read then?

[aueloxdad]

With the leads disconnected "OL" and with the leads connected and shorted "OL".

[bdunham7]

That's bad.  I don't have a Fluke 189 schematic and can't find a good, clear picture of the board, but if you are going to troubleshoot further you should find the TRMS converter chip.  I don't know which one it uses, but you can just look up each of the chips on the board until you find the one that is a TRMS converter.   Then measure the input and output of that chip to see what is going on there.

[BeBuLamar]

Just curious does it behave the same with the ACV as the ACmV?

[aueloxdad]

Yes it does.

[mwb1100]

There are a couple of (non-Fluke) documents that written up in Chinese for diagnosing and repairing some problems with 187/189 multimeters in this thread:

  - https://www.eevblog.com/forum/repair/fluke-187189-repair/msg3205856/#msg3205856

That particular post has the documents that have been auto-translated from Chinese to English.  The auto translation is dicey and can be hard to follow, but you might  be able to get some ideas for areas to look at. 

The one thing I could get from the first document (page 20) regarding AC reading problems is to visually check the custom Fluke 669918 chip which apparently handles the voltage and TRMS calculations.  The document says if that chip is bad the only repair is to replace it with a chip from another 187/189. I have no idea how you'd check out that chip more completely if it looks OK visually.  The 669918 chip also handles the DCV measurement (all measurements?), so if DCV is working I think it's likely not a problem with that chip.

The second document points out several points where the voltage can be measured to see make sure the power to various function blocks is OK.

[aueloxdad]

Thank you for your response and time.

[bdunham7]

the custom Fluke 669918 chip which apparently handles the voltage and TRMS calculations.  The document says if that chip is bad the only repair is to replace it with a chip from another 187/189. I have no idea how you'd check out that chip more completely if it looks OK visually.  The 669918 chip also handles the DCV measurement (all measurements?), so if DCV is working I think it's likely not a problem with that chip.

I wouldn't have thought that they would (or could) put a high-performance TRMS converter on their custom chip, but just disassembled mine and could not find any external TRMS converter.  So I guess that's that, unfortunately.

[Eraldo]

This multimeter can measure ac in 2 different modes: one in its own ac range switch and the other in the dc switch when pressing the blue button (ac+dc measurement) try measuring in this mode if it shows any value.

Then if it doesn't work you might want to check the white capacitor (near the movs and power resistors) to see if it works. Try measuring its capacitance with a multimeter or a simple transistor tester. Im not sure if you can get any meaningful value testing in-circuit in this case but give it a try.

There shouldn't be many things to go wrong in the ac if the dc works fine. Just to make sure measure some of the protection circuitry components and the fusible resistor.

Maybe I'm paranoid but definetly make sure the rotary switch contacts are intact and are in the correct orientation (check out some teardown images/videos) and make sure there are 4 of them.

Also test out the Ac milvolt range just to make sure. I believe it doesn't share the same path as the Vac and you can find out if the true rms converter works. You could generate a small signal with a function generator of even an arduino(make sure signal is  smaller than 5v and test it with another dmm)

[aueloxdad]

I will test it and get back to you.   Thanks

[Eraldo]

Some of the things i said above are a bit over the top and not all of them specific for your problem so i wanted to sum it up with this second post to be more clear and add some other things

1. Check all the basic ranges of the multimeters to see if any other range is broken/not working (Vac mvAc Vdc mvDc ohm currect ac current dc. Basic checks would suffice.

2. Check the mV AC range to see if the rms converter is fine. Even better if you check the current AC range since you'll be sure it goes directly to the rms converter instead of the voltage path which might be the thing compromised in this case

3. If these ac ranges work fine then you might need to check the main path of the ac line which is the same one as the dc one but instead of going straight to the resistor network (after passing throu the protection) ,it  passes through a capacitor first.

Check that capacitor whith another miltimeter by either measuring it's resistance, or better, try measuring it's capacitance. If you have another meter with capacitance measurement, try measuring it. The capacitor is a white package, as big as the protection resistors, which is found near the protection circuitry.

4. If all of them are ok then you might need to do some deep digging by yourself, testing the protection components etc

....5. This one is just a precaution but make sure you don't lose the contacts of the rotary switch when opening the multimeter.

Post pictures and other findings here and we'll help